1. Field of the Invention
The present invention relates to an ink jet print head including integrally arranged ejection ports from which ink is ejected, and a method of manufacturing the ink jet print head. More specifically, the present invention relates to the configuration of an ink jet print head which, when an elongate ink jet print head is used for printing, makes possible white or black stripes as unnoticeable as possible, the stripes being generated by fluctuation or meandering of the relative movement between the ink jet print head and a print medium.
2. Description of the Related Art
With the spread of copying apparatuses, information processing apparatuses such as word processors and computers, communication equipment, and the like, ink jet printing apparatuses printing digital images based on an ink jet scheme have been prevailing as output apparatuses printing images for the above-described apparatuses. The ink jet printing apparatus uses an ink jet print head with a plurality of integrally arranged ejection ports for printing. Techniques for such integral arrangement have been significantly improved in response to recent demands for higher resolution and high-speed output. Many full-line type ink jet printing apparatuses have also been provided which use an ink jet print head including a large number of densely arranged ejection ports corresponding to the width of print medium.
In a full-line type ink jet printing apparatus using an elongate ink jet print head, the elongate ink jet print head is fixed to the printing apparatus and ejects ink from individual ejection ports at a constant frequency as droplets. At the same time, a print medium is conveyed in a direction crossing the array direction of the ejection ports, at a constant speed corresponding to the ejection frequency and print resolution. That is, the operation of conveying only printing medium allows high-resolution images to be output at high speed.
For such an elongate ink jet print head, a method has been proposed which first manufactures a chip with a smaller number of ejection ports and combining a plurality of the chips together, in order to increase manufacturing yield.
FIG. 2 is a diagram showing arrays of ejection ports in an elongate ink jet print head disclosed in Japanese Patent Laid-Open No. 2005-199696. In FIG. 2, reference numerals 81 to 86 each denote a chip with two ejection port arrays. In the elongate ink jet print head 7, the chips 81 to 86 are consecutively arranged in the Y direction so as to be alternately staggered with respect to each other in the X direction.
The chips 81 to 86 have the same configuration and eject the same type of ink. For example, the chip 81 has an ejection port array 81A with ejection ports arrayed at a pitch of 600 dpi in the Y direction and an ejection port array 81B with ejection ports arrayed also at a pitch of 600 dpi in the Y direction. The two ejection port arrays are staggered with respect to each other by a half pitch (corresponding to 1,200 dpi). Thus, on a print medium conveyed in an X direction, dots can be printed at a resolution of 1,200 dpi. Such an elongate ink jet print head manufactured such that chips of the same type are staggered with respect to one another is hereinafter referred to as a “checker array print head”.
On the other hand, Japanese Patent Laid-Open No. 2005-199692 discloses a checker array print head with a plurality of chips arranged so as to form an overlap area in which the individual chips overlap one another in the Y direction.
FIG. 3 is a diagram showing arrays of ejection ports in two chips 90 and 91 in the checker array print head disclosed in Japanese Patent Laid-Open No. 2005-199692. According to Japanese Patent Laid-Open No. 2005-199692, in the overlap area in which the two chips 90 and 91 overlap, four ejection ports in each of the two chips are arranged at the same position in the Y direction. Japanese Patent Laid-Open No. 2005-199692 discloses a printing method in which the four ejection ports on each of the two chips alternately print one pixel line on a print medium conveyed in the X direction.
Specifically, print data arrayed in one line in the X direction is sorted into a plurality of ejection port arrays using a prepared mask pattern. In this case, for areas in which the two chips do not overlap, the print data is sorted into two arrays 94A and 94B or 94C and 94D. For the overlap area, the data is sorted into the four arrays 94A, 94B and 94C, 94D. The distribution rate for the sorting of the print data may be uniform or may vary among the ejection port arrays. Furthermore, for the overlap area, a mask pattern may be used which is made such that the distribution rate increases gradually from the ejection port at the end of the chip toward the center of the chip.
In the process of manufacturing a checker array print head, the arrangement of the individual chips inevitably involves a certain error. Black or white stripes may be observed in an image area printed via a boundary portion of each chip. However, in this case, such an overlap area as shown in the figure allows pixel lines arranged in the X direction and printed via the overlap area to be formed by four types of dots ejected from the two chips. That is, even if the two chips are slightly misaligned, an affect of the misalignment is prevented from concentrating at one position. Thus, a smooth boundary area with the possible black or white stripes made unnoticeable can be output. A method has also been proposed in which the amount of ink droplets ejected from ejection ports used to print the boundary portion is different from that of ink droplets ejected from the other ejection ports, in order to inhibit the image at the boundary portion from being degraded.
The checker array print head having two ejection port arrays in each chip has been described taking Japanese Patent Laid-Open Nos. 2005-199696 and 2005-199692 by way of example. However, the checker array print head need not necessarily include a plurality of ejection port arrays in each chip. The present specification considers any ink jet print head to be of the checker array type provided that a plurality of chips each with at least one ejection port array are consecutively arranged in the Y direction so as to be alternately staggered with respect to each other in the X direction. Any checker array print head configured as described above can exert such effects as disclosed in Japanese Patent Laid-Open No. 2005-199692.
However, in an ink jet printing apparatus using the checker array print head, negative effects on images associated with the accuracy with which the print medium is conveyed with respect to the ink jet print head have been acknowledged as problems. In particular, if a full-line type checker array print head in which individual ejection ports are densely arranged is used to print images on roll paper or the like at a high resolution of at least 1,200 dpi, possible meandering of the print medium has been determined to seriously affect output images. The negative effects on images caused by such meandering will be described below in detail.
FIG. 4 is a schematic diagram of a print head illustrating the negative effects associated with the meandering of the print medium. In FIG. 4, reference numerals 101, 102, and 103 denote three consecutive chips arranged in a checker array print head 100 and including ejection ports arrayed at a pitch of 1,200 dpi (a distance of 21 μm) The chips 101, 102, and 103 have an overlap area corresponding to four pixels. When the print medium is conveyed in a direction (X direction) perpendicular to an ejection port array direction (Y direction), dots printed via the first chip 101 and dots printed via the second chip 102 are regularly arranged at a pitch of 21 μm in the Y direction.
FIGS. 5A and 5B show print conditions and dot density distributions observed when the checker array print head shown in FIG. 4 is used. In FIG. 5A, one dot of diameter 35 μm is printed. In FIG. 5B, three dots are consecutively printed in the Y direction at a print resolution of 1,200 dpi. The single dot results in a density distribution with a peak located at the center of the dot as shown in FIG. 5A. When the plurality of dots are regularly arranged at intervals of 21 μm as shown in FIG. 5B, the density distribution includes a region with an almost uniform density value appearing consecutively in the Y direction.
Like FIGS. 5A and 5B, FIGS. 6A to 6E show dot print conditions and dot density distributions observed when the ink jet print head 100 is used. Each of FIGS. 6A to 6E shows the case in which the print medium is conveyed in a regular direction shown by a dash line in FIG. 4 and the case in which the print medium is skewed during the conveyance as shown by a solid arrow in FIG. 4. For description, the dots printed via the ejection ports in the chip 101 are shown by a pattern different from that for the dots printed via the ejection ports in the chip 102.
FIG. 6C is a diagram showing that the print medium is conveyed in the regular direction shown by the dash line in FIG. 4. The dot groups printed via the chips 101 and 102, respectively, are regularly arranged at intervals of 21 μm in the Y direction, similarly to the dots printed via the ejection ports in the same chip. Thus, the density distribution shows that an area is formed in which an almost uniform density value appears consecutively as in the case of FIG. 5B.
Now, with reference to FIG. 4, the case will be discussed in which the print medium is conveyed in a direction (−θ direction) angled with respect to the X direction. In this case, compared to the dots printed via the first chip 101, the dots printed via the second chip 102 are arranged at intervals larger than those (21 μm) corresponding to the print resolution. The intervals increase consistently with the skew of the print medium during the conveyance. That is, the dot print conditions and density distributions are as shown in FIGS. 6D and 6E. The density distributions show that an area with a lower density appears between the dot groups printed via the first and second chips 101 and 102, respectively.
On the other hand, when the print medium is conveyed in a +θ direction, the dots printed via the first and second chips 101 and 102 are arranged at intervals smaller than those (21 μm) corresponding to the print resolution. The intervals decrease with increasing skew of the print medium during the conveyance. That is, the dot print condition and density distribution are as shown in FIGS. 6A and 6B. The density distributions show that an area with a higher density appears between the dot groups printed via the first and second chips 101 and 102, respectively.
In contrast, the relationship between the chips 102 and 103 is reverse to that between the chips 101 and 102, described above. That is, an area with a higher density appears when the conveyance direction is skewed toward the −θ direction. An area with a lower density appears when the conveyance direction is skewed toward the +θ direction. As a result, if the conveyance direction of the print medium deviates from the regular direction, then in an output image, an area with a lower density and an area with a higher density appear alternately at a link of each chip. If the density value of the area with the higher density is larger than that of the other areas by at least a predetermined value, the area is viewed as black stripes. If the density value of the area with the lower density is smaller than that of the other areas by at least a predetermined value, the area is viewed as white stripes.
The negative effects of the skew of the print medium during the conveyance described above relate significantly to the difference between the two chips in the X direction. That is, as shown in FIG. 4, the amount of misalignment between the chips 101 and 102 (the distance, in the Y direction, between two dots printed via each of the chips 101 and 102) increases consistently with the distance L between the chips 101 and 102 in the X direction. Thus, if chips with more ejection port arrays in the X direction are prepared in order to achieve a higher print resolution, the distance between the ejection port arrays on the adjacent chips positioned on the opposite sides in the X direction may increase to further increase the amount of misalignment between printed dots.
The negative effects associated with the skew of the conveyance direction which is described above have not been successfully eliminated by the method disclosed in Japanese Patent Laid-Open No. 2005-199692. If an image is printed in an overlap area via different ejection ports in the respective chips as described in Japanese Patent Laid-Open No. 2005-199692, local white or black stripes are unlikely to appear. However, the density of the entire overlap area is lower than that of the other areas, resulting in a noticeable band-like unevenness.